Technical Field
The disclosure relates in generally related to a semiconductor device, and more particularly to an electrostatic discharge (ESD) protection device.
Description of the Related Art
ESD is a transient process of high energy transformation from external to internal of an integrated circuit (IC) when the IC is floated. Several hundred or even several thousand volts are transferred during ESD stress. Such high voltage transfer will break down the electronic devices of an input stage and cause circuit error. As the critical size of the IC is scaled down constantly, it is more and more important to provide a protected circuit or device to protect the electronic devices and to discharge ESD stress.
One solution to the problem of ESD, is to provide a device that is integrated into the IC for dispersing the ESD current into earth ground. For example, a gate grounded n-type metal-oxide-semiconductor (GGNMOS) with multiple fingers has been well known and can serve as an effective ESD protection device. The ESD protection provided by the GGNMOS is based on snapback mechanism. When the voltage reaches a level beyond the IC normal operation due to ESD zapping, the snapback mechanism enables the dingers of the GGNMOS to conduct a high level ESD current between its drain and source and subsequently directs the ESD current into the earth ground. As a result, the IC can be protected from being broken down by the ESD stress.
However, it has been reported that the multi-finger GGNMOS cannot be uniformly turned on under ESD stress. There are two main issues those responsible for the non-uniform turn-on problems of multi-finger GGNMOS. One is that the parasitic lateral n-p-n BJTs of center fingers usually have substrate (base) resistance greater than that of the fingers configure at the periphery under a uniform layout style, which makes the center-finger nMOS transistors be triggered on earlier. The other is the obvious snapback characteristics of the parasitic lateral n-p-n BJT inherent in the nMOS, which makes the first turned-on center fingers solely sustain the high ESD current. As a result, the conventional GGNMOS may be easily broken down, and thus permanent failure could be caused due to the uniform conduction.
Furthermore, the uniform conduction may be getting worse with the device critical size is continually scaled down. Therefore, there is a need of providing an improved ESD protection device to obviate the drawbacks encountered from the prior art.